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<br />not exist. For the unprotected area between the levee or flood wall and the source of <br />flooding, the elevations to be shown shall be obtained from either the flood profile that <br />would exist at the time levee overtopping begins or the profile computed as if the levee <br />did not exist, whichever is higher. <br />This procedure recognizes any increases in flood elevation that would occur in the <br />unprotected area that is caused by the levee or flood wall itself. This procedure may <br />result in flood elevations being shown as several feet higher on one side of the levee or <br />flood wall than on the other. Both profiles shall be shown in the final report and <br />labeled as "before levee overtopping" and "after levee overtopping" respectively. <br /> <br />Rule 10. Criteria for Determininl! Effects of Stormwater Detention on Rel!ulatorv Floodplains <br />Hydrologic determinations and increased runoff from development and urbanization shall be <br />considered through detention measures to mitigate the higher runoff characteristics. <br /> <br />A. Stormwater/Floodwater Runoff Detention. Hydrologic analyses may consider the <br />effects of on-site detention for rooftops, parking lots, highways, road fills, railroad <br />embankments, diversion dams, refuse embankments (e.g., solid waster disposal <br />facilities), mill tailings, impoundments, siltation ponds, livestock water tanks, erosion <br />control dams, or other structures only if they have been designed and constructed with <br />the purpose of impounding water for flood detention and are owned, operated and <br />maintained by a government body. Detention structures that are randomly located, <br />privately owned, or privately maintained shall not be included in the hydrologic <br />analysis unless they are determined to exacerbate downstream peak discharges. <br /> <br />B. Irrigation Facilities. The CWCB recommends that irrigation facilities (e.g., ditches and <br />canals) not be used as stormwater or flood conveyance facilities, unless specifically <br />approved and designated by local governing jurisdictions and acknowledged by the <br />irrigation facility owners. The flood conveyance capacity of irrigation facilities shall be <br />acknowledged only by agreement between the facility owners and local governing <br />jurisdictions. The CWCB may designate and approve 100-year floodplain information <br />for irrigation facilities ifthe above recommendations are met. <br /> <br />Rule 11. Effects of Flood Mitil!ation Measures and Stream Alteration Activities in Rel!ulatorv <br />Floodplains <br />In order to protect the health, safety, and welfare of the public, through the prevention or <br />reduction of flood hazards in Colorado, the CWCB shall recognize flood mitigation <br />measures and stream alteration activities, individually or collectively as the case may be, <br />provided that the following standards are met: <br />A. Floodwater detention/flood control storage facilities shall be designed and constructed <br />as part of a basinwide program for the watershed. <br />B. Detention facilities shall adequately consider flow rates and flow volumes for the 100- <br />year flood. <br />C. Major drainageway improvements and other flood control channelizations shall include <br />a low-flow channel with a capacity to convey the average annual flow rate, or other <br />appropriate flow rate as determined through a hydrogeomorphological analysis, without <br />excessive erosion or channel migration, with an adjacent overbank floodplain to convey <br /> <br />16 <br />